The Independent Police Complaints Commission investigation is separate from the over-arching judge-led inquiry Home Secretary Theresa May set up “to consider the extent to which state and non-state institutions have failed in their duty of care to protect children from sexual abuse and exploitation”.

The inquiry was established following claims that children had been abused by politicians, public officials and celebrities over many decades. In July 2014, Mrs May indicated that it would be able to study files held by MI5.

Former Liberal MP Cyril Smith, who died in 2010, was said to have abused children during the 1960s, 1970s and 1980s. In 2012, the Crown Prosecution Service said he should have been charged, while Greater Manchester Police said although he could not be convicted posthumously, “from the overwhelming evidence we have, it is right and proper we should publicly recognise that young boys were sexually and physically abused”.

His Labour successor alleges MI5 was involved in suppressing the allegations at the time. So what evidence is there that MI5 was involved in a cover-up, as alleged?

October 2012 – ‘powerful paedophile network’

In the Commons, Labour MP Tom Watson called on the police to examine allegations of a “powerful paedophile network linked to parliament and Number 10”. He did not name any individuals, but his comments sparked a wider examination about whether political figures were involved in abuse – and whether they used their connections to cover this up.

September 2013 – MI5 ‘held on to’ Smith evidence

Channel 4 Dispatches reported that the security services had held on to evidence of Smith’s abuse in Lancashire in the 1960s. Former Lancashire Special Branch detective Tony Robinson told Dispatches he was contacted by MI5, who requested he send them the file by special courier.

Smith’s Liberals propped up the Labour government in the late 1970s – the so-called Lib-Lab pact.

April 2014 – MI5 ‘put pressure on police’

Labour’s Simon Danczuk, a successor to Smith as MP for Rochdale, published a book, Smile for the Camera: The Double Life of Cyril Smith, saying MI5 and Special Branch put pressure on the police to drop their investigation into Smith’s activities.

Mr Danczuk first raised the Smith case in the Commons in 2012 after alleged victims contacted him.

July 2014 – abuse inquiry will see MI5 files

Mrs May implied that the judge-led inquiry she set up into historic child abuse would be able to see MI5 files.

In response to a question from Tom Watson, she said: “My intention is that the fullest possible access should be made to government papers in relation to these matters. Where there are files where there are certain issues around who can have access to those files we will need to ensure that we have an appropriate means of ensuring that the information is available to the inquiry panel.”

November 2014 – MI5 searches files

The Wanless report into whether there had been a cover-up of the Home Office’s handling of abuse allegations in the 1980s concluded that there was no evidence 114 missing official files had been “deliberately or systematically removed or destroyed to cover up organised child abuse”.

At the request of the Wanless inquiry, MI5 carried out a search of its own files, but said it had not come across anything relevant to the review. Mrs May told MPs: “I cannot stand here and say the Home Office was not involved in a cover-up in the 1980s and that is why I am determined to get to the truth of this.”

The inquiry was prompted by the hunt for a “dossier” handed to Leon Brittain in 1984, when he was home secretary, about alleged paedophile abuse in Westminster in the 1980s.

Lord Brittan, who died in January, was himself accused of being involved in the abuse of children in the 1980s. He always strongly denied the allegations

February 2015 – MI5 ‘knew about Kincora abuse’

Gary Hoy, who was abused at the Kincora children’s home in Belfast in the 1970s, was granted permission to launch a legal challenge against the government’s decision to exclude Kincora from its judge-led abuse inquiry.

He said MI5 knew about the abuse he and others suffered, but failed to do anything about it. The allegation is that by allowing the abuse to go on, the security services would be able to blackmail powerful paedophiles.

In 1981, three members of staff at the home were jailed for sexually abusing 11 boys.

March 2015 – agent ‘caught on camera’

BBC Newsnight said it had been told that a police operation into alleged abuse by Smith was scrapped shortly after he was arrested in the 1980s. According to an unnamed source, officers gathered evidence, including video footage, from a flat in south London. Smith was allegedly one of those caught on camera, along with a senior member of the intelligence agencies.

March 2015 – victims ‘threatened’ by MI5

Investigative journalist Don Hale (click on link for video) told Channel 4 News he had given the police evidence from interviews he had carried out with Smith and Brittan.

He said some victims and whisteblowers who had approached him had been “threatened” by police officers and MI5, “who’ve warned them not to talk to me or any other journalist and warned also that they might lose their pensions and about the officials secrets act and this is happening even up to the last few weeks”.

March 2015 – ‘intimidated’ by police

Kincora victim Richard Kerr (video below), who says he was trafficked from Belfast to be abused in London, told Channel 4 News police had “intimidated” him because they did not want him to give evidence at the 1981 trial.

Channel 4 News asked the Home Office for a response to the claims made about MI5 in this article, but has not received one.